In gallium nitride-type light emitting diodes (e.g. GaN, AlGaN, InGaN or AlInGaN light emitting diodes), the refractive index difference at the interface between the semiconductor and the air may result in a total reflection phenomenon. Due to the total reflection phenomenon, the viewing angle of the light emitting diode is restricted. Consequently, the researchers make efforts in developing a method for increasing the viewing angle of the light emitting diode.
For example, a light emitting diode package with a large viewing angle is disclosed in US Patent Publication No. 20090085053, which is entitled “Light emitting diode package with large viewing angle”. In this literature, a packaging technology is improved to widen the viewing angle of the light emitting diode package. FIG. 1 schematically illustrates a conventional light emitting diode package disclosed in US Patent Publication No. 20090085053. As shown in FIG. 1, the light emitting diode package comprises a substrate 30, a LED chip 32, a transparent housing body 33, a phosphor matrix 34, and an outer reflector 36.
The substrate 30 has an upper surface 38 and a lower surface 40. In addition, a first electrode 42 and a second electrode 44 are disposed on the substrate 30. The LED chip 32 is mounted on the upper surface 38 of the substrate 30. In addition, the LED chip 32 has a positive electrode 321 and a negative electrode 322. The positive electrode 321 of the LED chip 32 is connected with the first electrode 42 of the substrate 30 through a wire 46, and the negative electrode 322 of the LED chip 32 is connected with the second electrode 44 of the substrate 30 by the wire 46.
The transparent housing body 33 is disposed on the upper surface 38 of the substrate 30, and has a cavity 39. The LED chip 32 is disposed within the cavity 39. Moreover, the phosphor matrix 34 is inserted in the cavity 39. Consequently, the light emitted by the LED chip 32 may be transmitted through the phosphor matrix 34 and converted into white light. The white light may be transmitted through the lateral sides of the transparent housing body 33. The light transmitted through the lateral sides of the transparent housing body 33 is reflected by the outer reflector 36, so that the intensity of the light from the LED chip 32 is enhanced.
As described in this literature, the viewing angle of the conventional light emitting diode package is increased from 120 degrees to about 140 degrees.
Furthermore, a method of increasing side light extraction by the slant surface of the sidewall of the light emitting diode is disclosed in for example U.S. Pat. No. 6,570,190, which is entitled “LED having angled sides for increased side light extraction”. FIG. 2 schematically illustrates another conventional light emitting diode package disclosed in U.S. Pat. No. 6,570,190. As shown in FIG. 2, the LED comprises p-type and n-type doped epitaxial layers 10. The p-type and n-type layers 10 are arranged to provide a p-n junction region 11 (also referred as an active layer). A transparent substrate 12 and a window layer 13 are located at bilateral sides of the p-type and n-type doped epitaxial layers 10, respectively. A top electrical ohmic contact 14 and a bottom electrical ohmic contact 15 are attached on the surfaces of the transparent substrate 12 and the window layer 13, respectively. The area of the top surface 17 is larger than the area of the active layer 11. In addition, a sidewall 16 of the LED is oriented at an angle β relative to the vertical direction.
Moreover, since the sidewall 16 of the conventional LED has the slant surface, the laterally-transmitted light beams 18 and 19 may be subject to multiple reflection stages. The reflected light beams are directed to the top surface 17 of the LED, or reflected by the top surface 17 of the LED and emerged out of the sidewall.
Moreover, a sapphire substrate for forming a light emitting diode with a large viewing angle is disclosed in Solid-State Electronics 54 (2010) 509-515, which is entitled “Enhancement in emission angle of the blue LED chip fabricated on lens patterned sapphire (0 0 0 1)”. In this literature, a sapphire substrate with a special pattern is employed to form a light emitting diode with a large viewing angle.